The Properties and Generation of the Principal Winds

All the air has its course around the Earth from East to West.

This can be explained by describing the entire structure of the universe.

The eastern winds are usually much drier and make the air clearer and more serene than the western winds.

This is because the western winds oppose the usual course of vapors. These trap those vapors and cause them to thicken into clouds. Whereas the eastern winds push them away and disperse them.

• The eastern winds blow mainly in the morning.
• The western winds blow in the evening

This is because the Sun S:

• shines on the half ABC of the Earth.
• casts noon towards B and midnight towards D
• sets for people in A
• rises for people in C

The vapors near B are greatly expanded by the heat of the day. They flow:

• partly towards A
• partly towards C
• partly towards D where they occupy the space left by those condensed by the coolness of the night.

Thus, they create a west wind towards A, where the sun sets, and an east wind towards C, where it rises.

The east wind is usually stronger and faster than the west wind because:

• it follows the course of the entire mass of air
• the Earth between C and D has more condensation of vapors because it has had no sunlight longer than D and A

The north winds blow mainly during the day. They come from high to low and are very violent, cold, and dry.

This is because Earth EBFD is covered with several clouds and fogs towards the poles E and F, where it is hardly warmed by the sun, and towards B, where it shines directly, it stirs up a quantity of vapors.

These vapors are strongly agitated by the action of its light. They rise very quickly until they are so elevated that the resistance of their weight makes it easier for them to turn aside and take their course to both sides towards I and M above the clouds G and K, rather than continuing straight upward.

These clouds G and K are also heated and rarified by the sun at the same time, turn into vapors that flow from G to H and from K to L rather than towards E and F.

This is because the dense air towards the poles resists them much more than the vapors rising from the Earth towards the south, which, being very agitated and ready to move in all directions, can easily yield their place.

Thus, taking F for the Arctic Pole, the course of these vapors from K towards L creates a north wind, which blows during the day in Europe.

This wind blows downwards because it comes from the clouds towards the earth. It is usually very strong because it is stirred by the strongest heat of all, namely that of midday; and from the easiest material to dissolve into vapor, namely from the clouds.

This wind is very cold and very dry, partly because of its strength, following what was said above that impetuous winds are always dry and cold; It is also dry because it is usually composed only of the coarser parts of fresh water mixed with air, whereas humidity mainly depends on the finer parts.

These are hardly found in the clouds from which it originates; for, as you will soon see, they partake much more of the nature of ice than of water.

It is cold because it brings with it towards the south the very subtle matter that was towards the north, on which primarily the coldness depends. On the contrary, it is observed that the southern winds blow more often during the night, coming from below upwards, and are gentle and humid.

The reason for this can also be seen by looking again at the earth EBFD, and considering that its part D, which is under the Equator and where I suppose it is now night, still retains enough heat that the sun has communicated to it during the day to release several vapors; but the air above towards P does not retain as much in proportion.

For generally, coarse and heavy bodies always retain their heat longer than those that are light and subtle; and those that are hard also retain it longer than those that are liquid. This is why the vapors found towards P, instead of continuing their course towards Q and R, stop and thicken into clouds, which, by preventing those coming from the earth D from rising higher, force them to take their course to the sides towards N and O, thus creating a southern wind, which blows mainly during the night and comes from below upwards, namely from the earth towards the air; and it can only be very slow, both because its course is delayed by the density of the night air, and also because its material, coming only from the earth or water, cannot expand so rapidly or in such quantity as that of other winds, which usually come from clouds.

It is warm and humid, partly because of the slowness of its course; and it is humid because it is composed of the finest parts of fresh water as well as the coarsest, for they come out together from the earth; and it is warm because it brings with it towards the north the subtle matter that was towards the south.

In March, and generally throughout spring, the winds are drier, and the changes of weather more sudden and frequent, than in any other season of the year.

The reason for this is also seen by looking at the earth EBFD again and thinking that the sun, which I suppose is opposite the circle BAD representing the Equator, and was three months earlier opposite the circle HN representing the Tropic of Capricorn, has heated much less the half of the earth BFD where it is now spring, than the other half BED where it is autumn.

Consequently, this half BFD is much more covered with snow, and all the air surrounding it is much denser and filled with more clouds than that surrounding the other half BED; which causes during the day much more vapor to expand there, and conversely much more to condense during the night.

As the mass of the earth there is less heated and the force of the sun there is not less, there must be more inequality between the day’s heat and the night’s cold: and so these eastern winds, which I said blow mainly in the morning, and those from the north, which blow around midday, both of which are very dry, must there be much stronger and more abundant than in any other season.

Because the western winds, which blow in the evening, must also be quite strong there, for the same reason as the eastern ones that blow in the morning; provided the regular course of these winds is not advanced, or delayed, or diverted by the particular causes that can more or less expand or thicken the air in each region, they encounter each other and generate rains or storms, which usually cease soon after because the eastern and northern winds, which drive away the clouds, remain dominant.

These eastern and northern winds are what the Greeks called Ornithies, because they bring back the birds that come in spring.

But as for the Etesians, which they observed after the summer solstice, it is quite true that they proceed from the vapors that the sun raises from the lands and waters of the North, after having already spent quite a long time towards the Tropic of Cancer.

It lingers much more proportionally towards the Tropics than it does in the space between the two; and it must be thought that during the months of March, April, and May, it dissolves into vapors and winds most of the clouds and snows that are towards our Pole; but it cannot heat the lands and waters enough there to raise other vapors that cause winds until a few weeks later, when this great day of six months, which it has there, is a little beyond its midday.